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{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"Function's argument type inference is affected by the functions return type when in a comprehension","articleBody":"**Bug Report**\r\n\r\nMyPy seems to be getting confused if function that receives a generic class returns a union type with that generic is used in a comprehension. If the functions is used in a for loop or the function returns just the generic T, then MyPy doesn't show any errors.\r\n\r\n**To Reproduce**\r\n```python\r\nimport dataclasses\r\nfrom typing import Callable, TypeVar, Generic\r\n\r\nT = TypeVar('T')\r\n\r\n\r\n@dataclasses.dataclass(frozen=True, slots=True)\r\nclass Capsule(Generic[T]):\r\n    builder: Callable[[], T]\r\n\r\n\r\ndef process(argument: Capsule[T]) -\u003e T | RuntimeError:\r\n    if isinstance((value := argument.builder()), int) and value % 2 != 0:\r\n        return RuntimeError('Not an even number')\r\n\r\n    return argument.builder()\r\n\r\n\r\nencapsulated_ints = tuple(Capsule(builder=lambda: item) for item in range(10))\r\n\r\nrequests: list[int | RuntimeError] = []\r\nfor encapsulated_int in encapsulated_ints:\r\n    request = process(encapsulated_int)\r\n    requests.append(request)\r\nrequests = list(process(encapsulated_int) for encapsulated_int in encapsulated_ints)\r\n```\r\nError:\r\n```\r\nmain.py:25: error: Argument 1 to \"process\" has incompatible type \"Capsule[int]\"; expected \"Capsule[int | RuntimeError]\"  [arg-type]\r\n```\r\nhttps://mypy-play.net/?mypy=latest\u0026python=3.12\u0026gist=ffe7822cc564a6f347c2b8d6fd98c47f\r\n\r\n**Expected Behavior**\r\n\r\nThe list comprehension line should be interpreted exactly the same as the for loop.\r\n\r\n**Actual Behavior**\r\n\r\nMyPy somehow relates the function return to the generic type.\r\n\r\n**Your Environment**\r\n\r\n\u003c!-- Include as many relevant details about the environment you experienced the bug in --\u003e\r\n\r\n- Mypy version used: 1.9.0\r\n- Mypy command-line flags: no flags\r\n- Mypy configuration options from `mypy.ini` (and other config files): no configs\r\n- Python version used: 3.12\r\n\r\n\u003c!-- You can freely edit this text, please remove all the lines you believe are unnecessary. --\u003e\r\n","author":{"url":"https://github.com/ar-qun","@type":"Person","name":"ar-qun"},"datePublished":"2024-03-14T13:15:16.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":0},"url":"https://github.com/17025/mypy/issues/17025"}